The large-scale production of phosgene from CO and chlorine on activated carbon catalysts in a tube bundle reactor is known from the prior art (e.g. Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed. Vol. A 19p 413f., VCH Verlagsgesellschaft mbH, Weinheim, 1991). Carbon monoxide in stoichiometric excess is in this case combined with chlorine and passed over a fixed-bed catalyst which, with a particle size in the range of from 3 to 5 mm, is located in tubes having an inside diameter of from 50 to 70 mm. The carbon monoxide excess is required to keep the content of free chlorine in the phosgene as low as possible, because chlorine can lead to undesirable reactions in the subsequent use of phosgene.
In order to cool the highly exothermic reaction with an enthalpy of formation of −107.6 kJ/mol, a cooling medium is guided around the catalyst tubes. The reaction between CO and chlorine starts on the catalyst at about 40 to 50° C., the temperature in the tubes rising to about 600° C. and falling to 40 to 240° C. again at the reactor outlet. A phosgene quality is thereby obtained that satisfies the requirements of isocyanate production, for example. High starting material purity is required especially for the carbon monoxide because, after it has been combined with chlorine, contents of methane and hydrogen resulting from the production can lead to a highly exothermic reaction with the formation of hydrogen chloride. The temperature rise can lead to a dangerous reaction between chlorine and the material of the apparatus, the so-called chlorine/iron fire.
Phosgene is used in many areas of chemistry, either as an auxiliary substance or as an intermediate. The largest area of use in terms of quantity is the production of diisocyanates as starting materials for polyurethane chemistry. Particular mention may be made here of the substances 2,4- and 2,6-tolylene diisocyanate, the isomers of diphenylmethane diisocyanate and hexamethylene diisocyanate. Owing to the dissociation equilibria, phosgene at 100° C. already contains about 50 ppm chlorine. For many areas of use, such as, for example, the production of isocyanates for polyurethane production, such a chlorine content already represents the upper limit of the specification, as disclosed in EP 0 134 506 B1.
As a result, many attempts have been made in the past to improve phosgene production in terms of product quality and economy. Accordingly, EP 2067742 A1 describes a process for the production of phosgene with reduced CO emission, or reduced CO losses, by a main combination, subsequent condensation of the phosgene and then combination of the residual gas with chlorine. A phosgene having a low carbon tetrachloride content is obtained by using CO having a low methane content, as described in EP 1135329 B1. WO 2010/103029 A1 provides a process with a control concept for minimising the CO excess.
An important aspect in the production of phosgene is the reliable and uniform dissipation of the heat of reaction. This is achieved by a cooling medium being guided around the reaction tubes by forced convection or being partially evaporated around the tubes by natural convection. EP 0 134 506 describes a process in which cooling with both forced and free convection can be used to generate steam. In one example, a reactor with 415 tubes arranged in parallel is mentioned.
EP 1 640 341 describes a process in which natural convection is used for cooling. Water is employed as the cooling medium. Because the cooling medium must not enter the reaction chamber for reasons of corrosion, the pressure in the cooling medium must in principle be lower than in the reaction chamber, because leakages cannot be ruled out. EP 1 640 341 additionally describes an apparatus which comprises inter alia a tube bundle reactor and a separator for the partially evaporated cooling medium. For the better cooling of large reactors, flow deflectors (baffle plates) are described. In this arrangement, however, it is in some cases perceived as a disadvantage that the number of tubes is limited in practice to about 3000 tubes.
WO 2010/076209 A1 describes a reactor which is equipped on the inside with baffle plates for a liquid heat exchanger medium. A more uniform heat dissipation is thereby said to be achieved in the case of large reactor diameters and increased throughputs, without increasing the amount of heat exchanger medium in circulation. The process is said to be suitable in particular for reactors having 1000 to 3500 contact tubes.
WO 2010/076208 A1 is concerned with a similar approach, that process being said to be suitable in particular for reactors having 2000 to 6000 contact tubes with direct cooling.
A further phosgene reactor with increased capacity is known from EP 1 485 195 A1. In that reactor, baffle plates for the cooling medium are arranged in such a manner that the cooling medium is guided in alternating sections perpendicularly to the tubes filled with catalyst. In addition, for reasons relating to the flow, no reaction tubes are arranged in some areas. Here too, the number of reaction tubes is generally between 1000 and 3500.
The necessity of producing ever greater capacities in an economical manner means that multiple lines are to be avoided and, instead, the flow rates through the single reactor are increased ever further. A problem that arises therein is the necessity to reliably cool each individual tube in a tube bundle reactor. Evaporative cooling by free convection, which is an efficient process, can be used for that purpose. However, adequate cooling by free convection requires each individual tube to be supplied with sufficient cooling medium over the entire heated length. In the case of starting materials that flow into the tube bundle reactor from the bottom, that is particularly challenging in the region immediately above the bottom tube plate, because a comparatively large amount of heat has to be dissipated there.